Fiber specklegram sensor based on reflective mode rotation in few-mode fiber

Abstract

In this paper, a fiber specklegram sensor is formed from reflective mode rotation in few-mode fiber (FMF) with tilted fiber Bragg gratings (TFBG). The FMF-TFBG with different tilt angles have been fabricated, and could realize the high-efficient reflective mode conversion from fundamental mode to high-order core modes (LP11, LP21, LP02, LP31, and LP12 modes) simultaneously when the tilt angle is 1.6°. With the twist induced in FMF-TFBG, the recorded LP11, LP21 and LP31 mode specklegram rotated with fiber twisting. The specklegrams rotation angle are detected using speeded-up robust feature algorithm combined with Canny edge detection algorithm to improve the accuracy. When fiber twist angle changes from −300° to 300°, the specklegram rotation angle of LP11, LP21 and LP31 mode is linearly related to fiber twist angle, with linearity of 0.515, 0.544 and 0.528, respectively. Therefore, the proposed mode rotation from FMF-TFBG with advantages of low cost, high accuracy and excellent performances, could give rise a new generation of fiber specklegram sensor.